The top tube size indicates a standard model, my number was under the bottom bracket and had 84 in it. That seems to indicate an 1984 bike. I could be wrong but if your top tube is flat and not sloped im sure its before 1984, its just that all the GTs ive found on the net are like this before 84. Ive had the same trouble tracking my frame it seems its a standard 84 frame with pre 83 forks. Its in this resto section somewhere.